1. Field of the Invention
This inventing relates to pharmaceutical formulations containing kappa agonist compounds and to their medical use as anit-pruritic agonist at kappa opioid receptors.
2. Reported Developments
A) Antihyperalgesic Kappa Agonists
Opium and its derivatives are potent analgesics that also have other pharmacological effects, and exert their effects by interacting with high-affinity receptors.
It has been shown by investigators that there are at least three major opioid receptor types in the central nervous system (hereinafter CNS) and in the periphery. These receptors, known as mu (.mu.), delta (.delta.) and kappa (.kappa.), have distinct pharmacological profiles, anatomical distributions and functions. [See, for example: Wood, P. L., Neuropharmacology, 21, 487-497, 1982; Simon, E. J., Med. Res. Rev., 11, 357-374, 1991; Lutz et al, J. Recept. Res. 12, 267-286; and Mansour et al, Opioid I, ed. Herz, A. (Springer, Berlin) pp. 79-106, 1993.] The .delta. receptors are abundant in CNS and mediate analgesia, gastrointestinal motility and various hormonal functions. The .mu. receptors bind morphine-like drugs and mediate the opiate phenomena associated with morphine, including analgesia, opiate dependence, cardiovascular and respiratory functions, and several neuroendocrine effects.
The .kappa. receptors have a wide distribution in CNS and mediate a spectrum of functions including the modulation of drinking, water balance, food intake, gut motility, temperature control and various endocrine functions. They also produce analgesia. [See, for example: Leander et al, J. Pharmacol. Exp. Ther. 234, 463-469, 1985; Morley et al, Peptides 4, 797-800, 1983; Manzanares et al, Neuroendocrinology 52, 200-205, 1990; and Iyengar et al, J. Pharmacol. Exp. Ther, 238, 429-436, 1986.]
Most clinically used opioid analgesics such as morphine and codeine act as .mu. receptor agonists. These opioids have well-known, undesirable and potentially dangerous dependence forming side effects. Compounds which are .kappa.-receptor agonists act as analgesics through interaction with .kappa. opioid receptors. The advantage of these agonists over the classical .mu. receptor agonists, such as morphine, lies in their ability to cause analgesia while being devoid of morphine-like behavioral effects and addiction liability.
B) Antihyperalgesic Kappa Agonists as Anti-Pruritic Agents
The prior art has investigated the physiology and treatment of pruritus as illustrated hereunder.
Itch is a well known sensory state associated with the desire to scratch. As with pain, itch can be produced by a variety of chemical, mechanical, thermal or electrical stimuli. In addition to the difference in the sensory quality of itch and pain, they also differ in that (1) itch, unlike pain, can only be evoked from the superficial layers of skin, mucosa, and conjunctiva, and (2) itch and pain usually do not occur simultaneously from the same skin region; in fact, mildly painful stimuli, such as scratching, are effective in eliminating itch. In addition, the application of histamine to skin produces itch but not pain. Itch and pain are further dissociated pharmacologically: itch appears to be insensitive to opiate and non-steroidal anti-inflammatory drug (NSAID) treatment, both of which are effective in treating pain.
Although itch and pain are of a class in that both are modalities of nociception transmitted by small unmyelinated C fibers, evidence that itch is not just a variety of low-threshold pain is overwhelming. Itch leads to the reflex or urge to scratch; pain leads to withdrawal. Removal of the epidermis eliminates itch but causes pain. Analgesics, particularly opioids, relieve pain but often cause itch (see, for example J. Am. Acad. Derm. 24:309-310, 1991). There can be no doubt that itching is of eminent clinical importance; many systemic and skin diseases are accompanied by persistent or recurrent itch attacks. Current knowledge suggests that itch has several features in common with pain but exhibits intriguing differences as well (see, for example, W. Magerl, IASP Newsletter, pp. 4-7, September/October 1996).
McMahon et al (TINS, Vol. 15, No. 12, pp. 497-501, 1992) provides a description of stimuli (Table a) and a comparison of the established features of itch and pain (Table b):